This invention relates to magnetic lamp ballasts and especially to improved ballasts having an amorphous metal core structure which has a dual function as a capacitor.
Fluorescent and mercury vapor lamps require special circuitry for their starting and running when excited from an alternating current supply. These lamps have a negative resistance characteristic which must be compensated by ballasting impedance, and the ballast supplies a higher peaked voltage for starting and a regulated square wave current for running. It is desirable that the current through the lamp be flat-topped to increase the life of the lamp. A high reactance transformer is needed to meet the requirements of a good ballast, and a capacitor is added to the ballast circuit for starting and power factor correction. A typical ballast for two fluorescent lamps (see FIG. 2) has a relatively small starting capacitor and a large power factor capacitor, both discrete components separate from the high reactance transformer. Present magnetic ballasts are made from steel lamination punchings and include mangetic shunts and cutaways to cause core saturation. The present configurations substitute cores made from amorphous metal ribbon for the lamination punchings.
Amorphous metal is also known as metallic glass and is made from metallic alloys that can be quenched rapidly without crystallization. The material is fabricated on a rotating chill cylinder in the form of a long ribbon with a thickness of 2 mils or less; the thickness limitation is set by the rate of heat transfer through the already solidified material which must be rapid enough that the last increment of material to solidify still avoids crystallization. This is several times thinner than currently used lamination materials. Despite this possible limitation, at power frequencies amorphous metal core material is attractive because of the combination of potential low cost and low magnetic losses; the core loss is about one-fourth the loss found in silicon sheet steel.
It has been recognized that the thinness of amorphous metal ribbon can be capitalized upon by utilizing the stator or rotor core laminations as plates of a start/run capacitor in a single phase electric motor. This integral construction is made possible by the tremendously increased interlamination area with the thinner material, and is described in allowed application Ser. No. 914,444 filed on June 12, 1978, T. H. Haller, "Amorphous Metal Electric Motor with Integral Capacitor". Amorphous metal "0" core ballasts and reactors having magnetic structures made of ribbon without interlaminar insulation are disclosed in application Ser. No. 966,855, R. P. Alley and R. E. Tompkins, filed on Dec. 6, 1978. Both are assigned to the instant assignee.